1. Field of the Invention
The present invention relates to cardiac defibrillating and pacing systems, and in particular to implantable systems of this type employing a patch electrode.
2. Description of the Prior Art
Many types of implantable systems are known for tachyarrhythmia control. Such systems have gained greater acceptance in recent years as an alternative therapy to chronic pharmacologic treatment. Such tachyarrhythmia control systems typically include an implantable device capable of tachyarrhythmia detection and delivery of an automatic therapeutic response to the arrhythmia, including bradycardia pacing support, anti-tachyarrhythmia pacing, low energy synchronized cardioversion or high energy defibrillation shock, an electrode system for sensing and pacing, and a high energy electrode system for delivery of defibrillation shock. Typically the pacing and sensing electrode system will consist of a bipolar endocardial lead or two unipolar myocardial leads. The high energy electrode system generally consists of two myocardial patches or a transvenous shocking electrode and a myocardial or subcutaneous patch.
Any device which is intended to provide automatic treatment of ventricular tachyarrhythmias must be capable of first detecting the presence of such arrhythmias prior to the onset of therapy. Several methods are known for detecting ventricular tachyarrhythmias. These include monitoring an absolute heart rate interval, and initiating therapy when the interval becomes less than a programmable interval threshold. It is also known to try to differentiate pathologic rhythms from normal physiologic rhythms by analyzing the rate of onset (sudden change, as opposed to gradual change in the heart rate interval) and/or heart rate stability. It is also known to determine the probability density function of a signal corresponding to heart activity, which involves the evaluation of the time which the cardiac electrical signal spends at an isoelectric base line, and to initiate therapy when deviations beyond a predetermined threshold occur.
The known detection techniques have several limitations and disadvantages. The two major disadvantages are (1) no accurate method of differentiating between a pathologic (i.e. hemodynamically compromising) rhythm versus a physiologic (i.e., sinus) rhythm, and (2) total reliance on a processed electrogram for detection of cardiac depolarization. As a result of the second disadvantage, certain rhythms, particularly low amplitude ventricular fibrillation, may not be detected. These limitations in the known detection techniques may result either in a false positive detection response (inappropriate shock delivery) or a false negative detection response (failure to respond to a pathologic rhythm).